In recent years, many different techniques for the fast production of three-dimensional models have developed for industrial use, which are sometimes referred to as Rapid Prototyping and Manufacturing (RP&M) techniques. In general, RP&M techniques build a three-dimensional object, layer-by-layer, from a working material utilizing a sliced data set representing cross-sections of the object to be formed. Typically an object representation is initially provided by a Computer Aided Design (CAD) system.
Stereolithography, the presently dominant RP&M technique, may be defined as a technique for automated fabrication of three-dimensional objects from a fluid-like material utilizing selective solidification of thin layers of the material at a working surface to form and adhere successive layers of the object (i.e. laminae). In stereolithography, data representing the three-dimensional object is input as, or converted into, two dimensional layer data representing cross-sections of the object. Thin layers of material are successively formed and selectively transformed (i.e., cured) into successive laminae according to the two-dimensional layer data. During transformation the successive laminae are bonded to previously formed laminae to allow integral formation of the three-dimensional object.
A preferred material typically used in a Stereolithographic Apparatus (SLA) is a liquid photopolymer resin. Typical resins are solidifiable in response to selected wavelengths of electromagnetic radiation (e.g., selected wavelengths of ultraviolet (UV) radiation or visible light). This radiation of selected wavelength may be termed "solidifying radiation". The electromagnetic radiation is typically in the form of a laser beam which is directed to a target surface of the resin by way of two orthogonal computer controlled scanning mirrors. The scanning speed of the beam across the liquid surface is controlled so as to provide a desired exposure and associated depth of cure. A more detailed description of stereolithographic techniques (i.e. methods and apparatus) is found in the following patents and applications which are hereby incorporated by reference:
U.S. Pat. No. 4,575,330 to Hull:
Describes the fundamentals of stereolithography.
U.S. Pat. No. 5,058,988 to Spence et al.:
Describes the use of beam profiling techniques in stereolithography.
U.S. Pat. No. 5,059,021 to Spence et al.:
Describes the use of scanning system drift correction techniques for maintaining registration of exposure positions on the target surface.
U.S. Pat. No. 5,104,592 to Hull et al.:
Describes the use of various scanning techniques for reducing curl-type distortion in objects that are being formed stereolithographically.
U.S. Pat. No. 5,123,734 to Spence et al.:
Describes a technique for calibrating a scanning system on a stereolithographic apparatus.
U.S. Pat. No. 5,133,987 to Spence et al.:
Describes the use of a large stationary mirror in the beam path between the scanning mirrors and a target surface.
U.S. Pat. No. 5,182,056 to Spence et al.:
Describes the simultaneous use of multiple wavelengths to expose the resin.
U.S. Pat. No. 5,184,307 to Hull et al.:
Describes the use of slicing techniques for converting three-dimensional CAD data into cross-sectional data for use in exposing the target surface to appropriate stimulation.
U.S. Pat. No. 5,321,622 to Snead et al.:
Describes the use of Boolean operations in deriving cross-sectional data from three-dimensional object data
U.S. Pat. No. 5,999,184, to Smalley et al.:
Describes the use of solidification techniques to simultaneously cure multiple layers.
U.S. Pat. No. 5,965,079, to Gigl et al.:
Describes various scanning techniques for use in stereolithography.
Commercially available photopolymers for use in Stereolithography are typically of acrylate, epoxy or combined chemistry. Typically, resins contain a plurality of components. These components may include one or more photoinitiators, monomers, oligomers, inert absorbers, and other additives. The usefulness of resins for stereolithography is in part determined by the photospeed of the resin and the ability of the resin to form adequately cohesive laminae of appropriate thickness. It is desired that the photospeed be high enough to enable rapid solidification of cross-sections with available power levels of solidifying radiation. Further, since the depth of polymerization in the resin is linked to the locations at which photons are absorbed, absorption of photons by the resin must be sufficient to form adequately thin layers. Examples of preferred photopolymers include, but are not limited to, SL 5170, SL 5180, SL 5081, SL 5154 and SL 5149 (manufactured by Ciba Specialty Chemicals Corporation North America of Los Angeles, Calif. and as sold by 3D Systems, Inc. of Valencia, Calif.), SOMOS 6100, 6110, 5100, 5110, 2100 and 2110 (manufactured by Du Pont Company, New Castle, Del.).
The photoinitiators are the component of the resin that determines the photosensitivity of the resin at a given wavelength. Radiation absorption by the photoinitiator leads to chemical changes in the photoinitiator which can cause polymerization of the monomers and oligomers. Thus, radiation of appropriate wavelengths to be absorbed by the photoinitiator is known as solidifying radiation. At some wavelengths the monomers/oligomers can absorb electromagnetic radiation. As absorption by the monomers/oligomers typically do not yield an efficient polymerization reaction, the absorption of solidifying radiation by the monomers/oligomers is typically undesired. Thus, the most effective wavelengths for use in stereolithography are those which are strongly absorbed by the photoinitiator (high coefficient of absorption) and only weakly absorbed by the monomers and oligomers (low coefficient of absorption). Examples of preferred photoinitiators include, but are not limited to, triarylsulfonium salts, mixtures of triarylsulfonium salts with phosphate salts or antimonate Salts; 2,2-dimethoxy-2-phenyl acetophenone (BDK) C 16H.sub.16 O.sub.16 ; 2,4,6-trimethyl benzoyl diphenyl phosphine oxide (TPO); and 1-hydroxycyclohexyl phenyl ketone (HCPK) C.sub.13 H.sub.16 O.sub.2.
The useable wavelength range is bounded at the low wavelength end by monomer/oligomer absorption properties and at the upper wavelength end by photoinitiator absorption. As such, the reactive (i.e., actinic) spectral sensitivity of a photopolymer resin may be described as the product of the photoinitiator absorption spectrum and the monomer/oligomer transmission spectrum, as shown in FIG. 1. FIG. 1 depicts plots of photoinitiator absorption 11, monomer/oligomer transmission 13, and reactive sensitivity or reactive response 15 of the resin. As the absorption and transmission coefficients not only depend on the specific chemical composition of each component, but also on the concentrations of each component within the resin, shifts in wavelength for peak reactive response may result due to changes in either composition or concentration. For a given resin composition this peak can be readily determined by one of skill in the art.
In other words, the absorption by the monomer/oligomer, which depends upon the wavelength of radiation, affects the effectiveness of activitating the photopolymers as, in some instances, monomer/oligomer absorption competes with the absorption by the photoinitiator.
In the example of FIG. 1, the peak reactive response occurs within a range of about 328 nm-337 nm and the half maximum response falls within the range of about 320 nm to about 345 nm. As such, in this example electromagnetic radiation within the range of 320 to 345 nm is preferred and electromagnetic radiation within the range of 328 to 337 nm is even more preferred. The more preferred range includes the wavelengths which are within 10% of the peak reactive response. The preferred range includes wavelengths which are within 50% of the peak reactive response.
Until recently, all commercial Stereolithography systems used helium-cadmium (HeCd) lasers which emit radiation at 325 nm or argon-ion (Ar.sup.+) lasers which emit radiation primarily at 351 nm. Table 1 illustrates key characteristics of present lasers.
TABLE 1 Laser Input Emission Output Output Typical Type Power Wavelength Power Type Lifetime HeCd &lt;1 kW 325 nm 50-70 mW CW 5000 hrs Ar.sup.+ 20-30 kW 351 nm 300-500 mW CW 4000 hrs Present &lt;1 kW 355 nm 300-500 mW Pulsed 5000 hrs DPSS
Helium-cadmium lasers have a wavelength, input power and output type which are suitable and acceptable for stereolithography. However, the output power from these lasers is very limited and unsuitable where large objects or faster build speeds are needed. Thus, although HeCd lasers are useful in Stereolithography, they do not achieve all of the needs of Stereolithography.
Argon-ion lasers have output power levels and output modes which are suitable for faster part building and/or larger Stereolithography parts. However, the input power is excessive, and necessitates the need for water cooling. Further, the emission wavelength is outside the preferred range and the lifetime is undesirable.
Present diode pumped solid state (DPSS) lasers have both input and output powers which are suitable for stereolithography. These lasers are pulsed in contrast to the prior gas lasers (e.g., HeCd and Ar.sup.+) which provided a continuous wave (CW) laser beam. However, to effectively utilize these lasers a sufficiently high pulse repetition rate is needed to ensure that continuous cured lines of photopolymer are formed. Further, the emission wavelength of present DPSS lasers, though presently used in stereolithography, is outside the preferred range.
Recently, some commercial stereolithographic systems have been employed using pulsed solid state lasers to selectively solidify the material. These commercial systems have employed the use of frequency tripling to bring the 1064 nm fundamental infrared radiation of Nd:YVO.sub.4 pulsed solid state lasers into the ultraviolet range. This frequency tripling has resulted in a wavelength of 355 nm. The effectiveness of this laser for use in stereolithography has been limited due to its output wavelength being outside both the more preferred range (i.e., 328-337 nm) and the preferred range (i.e., 320-345 nm).
Considering the preferred and more preferred wavelength ranges for stereolithography, as based on the example of FIG. 1, and considering that solid state lasers do not directly emit in these ranges, frequency multiplication of fundamental and secondary (i.e., wavelength resulting from first frequency multiplication) wavelengths are necessary to obtain the desired output wavelengths. Working backwards one can obtain the necessary fundamental wavelengths. To obtain wavelengths in the more preferred range one would need to start with wavelengths in the ranges of:
 For frequency doubling 656-674 nm, For frequency tripling 984-1011 nm, For frequency quadrupling 1312-1348 nm.
To obtain wavelengths in the preferred range one would need to start with wavelengths in the ranges of:
 For frequency doubling 640-690 nm, For frequency tripling 960-1035 nm, For frequency quadrupling 1280-1380 nm.
Thus, to obtain desired output wavelengths, one must start with an appropriate solid state lasing crystal with an output wavelength in one of the above ranges. In addition to starting with the appropriate wavelength, one must also consider other characteristics of the fundamental output of the lasing crystal such that effective conversion can occur and desired average power and pulse repetition rate is obtained. Such characteristics include pulse duration (i.e. the time width of the pulses that are produced), the emission cross-section of the crystal, and the excited state lifetime. Although sufficiently high power lasers of appropriate fundamental wavelength exist and are commercially available, frequency multiplying of these wavelengths within the preferred or more preferred wavelength range and at appropriate repetition rates has not been demonstrated.
Another factor to be considered is related to the scanning speed of the laser. For the cured line to be continuous and absent large modulations, the time between the laser pulses must be such that the solidified material overlaps or, at a minimum, meets.
For the foregoing reasons, a need exists in the stereolithography art for an efficient, long lived laser operating within the preferred or more preferred wavelength range, at power levels above 100 mW, and preferrably above 300 mW, and operating at appropriate pulse repetition rates. It is desired that such a laser operate with an input power below 1 kW, an output power above 300 mW, an emission wavelength between 320 nm and 345 nm, and more preferrably between 328-337 nm, a pulse repetition rate which depends on laser power but typically exceeds 20 kHz, and a lifetime above 10,000 hours.